A typical synchronizer ring, which has conventionally been used in a synchromesh type of gear reducer (hereinafter referred to as “transmission”) for automobiles, is an annular sliding friction part for allowing two gears to smoothly engage with each other and to rotate synchronously by sliding at the time of gear change in a transmission of automobiles.
Such synchronizer ring is generally made of brass (a Cu—Zn alloy). Its inner surface to be in contact with the gear cone has annular streak grooves for giving friction force and vertical grooves for releasing lubricating oil, as needed.
It is also known that a material with high melting point such as molybdenum and ceramics is thermally sprayed on the inner surface to form a sprayed coating layer, which is fixed onto the body of the synchronizer ring.
There is also a known method for improving its friction characteristics, which includes the step of fixing, onto its inner surface, a layer of a friction material that contains a resin agent as a binder, a carbon material, a thermosetting resin, a metallic material, an inorganic material, and the like. For example, such a method is disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 57-195923, 59-187114, 09-79288, 09-221553, and 11-61103. In the disclosures, the carbon material includes wood powder, cotton and graphite. The above disclosures are, however, completely silent on the use of petroleum coke or pitch coke of the present invention as well as general cokes. JP-A-05-32955 discloses a wet friction material consisting of 10 to 30% by weight of graphite powder, 3 to 15% by weight of coke powder having a porosity of 10 to 20%, and the remaining part of a calcined powder composed mainly of copper. However, this reference does not describe anything about not only cokes with low ash content but also wet friction materials using a thermosetting resin. High performance engines or transmissions developed in recent years are requiring high coefficient of dynamic friction of the synchronizer ring for not only their reliable operation but also for good shift feeling in the operation. Of course, since the synchronizer ring is a sliding friction part, high resistance to abrasion and seizing is required. In the operation of such a high performance engine, an inadvertent shift of the shift lever without sufficient depression of the clutch pedal or a mistaken up- or down-shift can cause the synchronizer ring to generate a greater frictional heat than that in the conventional case and thus greater heat load. Therefore, the synchronizer ring should have higher resistance to abrasion and seizing.
For the purpose of increasing the frictional force, for example, the pressing force of the synchronizer ring may be mechanically increased, or the shape of the inner surface may be changed for higher contact pressure. In such a case, however, the brass synchronizer ring can generate a relatively great heat load when brought into contact with the counterpart gear cone, so that its synchronization performance can be reduced by the action of frictional heat.
Against such technical problem, there is a method for increasing the capacity for absorbing the frictional heat and improving the synchronization performance, which includes providing two, three or more frictional surfaces for the brass synchronizer ring to form a multi-cone type synchronizer ring. Such method, however, would increase the number of parts for the synchronizer ring and can lead to higher cost thereof and larger transmission size responsible for an increase in the weight of the vehicle.
It is also known that the method of thermally spraying the refractory material such as molybdenum on the inner surface can produce an abrasion or seizing resistance for withstanding the frictional heat but cannot produce high coefficient of dynamic friction at the same time.
Against such a technical problem, for example, Japanese Patent No. 2992373 discloses a technique of compounding and dispersing a hard ceramic into molybdenum or the like, followed by thermal spraying. Concerns remain, however, that the scratching action of the ceramic at the time of frictional sliding can cause significant abrasion of the sliding surface of the gear cone, which is generally made of steel, so that the synchronization performance or function can be reduced or degraded.
Any friction material containing the binder of the resin agent as disclosed in the above patent publications has an improved coefficient of dynamic friction, an improved resistance to abrasion or seizing, or an improved friction coefficient at a relative velocity of near-zero rpm (coefficient of static friction), compared with the brass for the conventional brass synchronizer ring. Such a material, however, is not necessarily satisfactory, and a further improvement in performance has been desired.
Thus, there has been a demand for development of a friction material for a synchronizer ring so as to form a good friction material layer on the inner surface of the synchronizer ring. The present invention has been made intensively to solve the above problem.